1. Field of the Invention
The present invention relates generally to audio equipment which can always obtain an appropriate sound characteristic without being affected by ambient noises.
2. Description of the Prior Art
When listening to reproduced sound in a noisy environment, a listener often cannot clearly hear the reproduced sound due to the high ambient noise level.
For example, in the case of stereo equipment installed within an automotive vehicle compartment, the reproduced sound becomes difficult to resolve due to the relatively high noise level within the compartment while the vehicle is running even though the output volume of the stereo can be appropriately adjusted while the vehicle is at rest.
A conventional audio system which solves the above-described problem has been reduced to practice. The contents of the conventional audio equipment is described in "NISSAN SERVICE WEEKLY REPORT No. 428 (LP-1)" Pages 270 and 271 published by Nissan Motor Company Limited on September, 1980. In this publication, the conventional audio equipment monitors the ambient noise level, checks the position of a fan speed changeover switch of an air conditioner, and automatically controls the output volume level of an amplifier in the audio system according to these conditions so that the reproduced sound can be easily resolved despite a high ambient noise level.
However, there is a problem in the above-described conventional audio equipment in that since the output sound level varies only according to the ambient noise level, the output sound in frequency bands which are easy to resolve is amplified together with other frequency bands which are more difficult to resolve so that the overall output volume may become too loud. In the worst case, the output sound will be distorted to a displeasing extent.